An amplifier stage operates in class A if the included amplifying elements, usually transistors, are biassed so that they operate within their linear (active) range for all the input signals to the amplifier. In a class B amplifier stage, the included transistors are biassed so that they are controlled to full conductive or non-conductive stage for input signals of given polarity. In such amplifier stages where the input signal is sinusoidal, for example, a transistor is controlled to conductive stage during one half-period of the input signal, while the transistor is blocked during the other half-period. A second transistor is therefore provided in the amplifier, which is controlled to a conductive stage during the second half-period, so that am amplified output signal is obtained during both half-periods of the input signal.
A class AB amplifier stage operates substantially as a class B amplifier stage. In a range around zero where the load current reverses, however, both amplifying elements are conductive in class AB. The current flowing through both amplifying elements when the load current is zero is designated zero current, or quiescent current. Previously known amplifier circuits for controlling the quiescent current are described in, for example, the German "Offenlegungsschrift" Nos. 22 52 666 and 26 14 399.